Olivier Touret

Reduction of CeO2 by hydrogen. Magnetic susceptibility and Fourier-transform infrared, ultraviolet and X-ray photoelectron spectroscopy measurements

The reduction of CeO2 by hydrogen has been studied from 300–1200 K by several complementary techniques: temperature-programmed reduction (TPR) and magnetic susceptibility measurements, Fourier-transform infrared (FTIR), UV–VIS diffuse reflectance and X-ray photoelectron (XP) spectroscopy. Two CeO2 samples were used with B.E.T. surface areas of 115 and 5 m2 g–1, respectively. The concentration of Ce3+ was determined in situ by measuring the magnetic susceptibility and the CeIII photoemission line. The reduction began at 473 K, irrespective of the initial surface area of the ceria. In the case of the low-surface-area sample, an intermediate reduction step was observed between 573 and 623 K, corresponding to the reduction of the surface. This intermediate step was less easily observed in the case of the high-surface-area ceria. In both cases, the reduction led to a stabilised state with the formal composition CeO1.83. Temperatures higher than 923 K were required to reduce the ceria further. The surface CeIII content determined by XPS was close to that determined by magnetic susceptibility measurements. The intensity of the 17 000 cm–1 band in the UV–VIS reflectance spectrum also varied with the degree of reduction. Finally, the evolution of the surface species observed by IR spectroscopy was in good agreement with the results from the other techniques. The IR results indicated large changes in the concentration and nature of both the hydroxyl and the polydentate carbonate species during the reduction process. The adsorption of oxygen on samples previously reduced to the composition CeO1.83 led to almost complete reoxidation at room temperature. The state of the initial B.E.T. surface did not influence the oxidation process. A slight excess adsorption of oxygen was evident on the surface. This was thermodesorbed at 380 K under vacuum.

THERMAL STABILITY OF DOPED CERIA : EXPERIMENT AND MODELLING

The process of the decrease of the surface area due to crystallite growth in ceria at 943 K is described by a kinetic model involving oxygen and cerium diffusion. The experimentally found variations in the rate of crystallite growth are reported as a function of the content (⩽ 10% cat.) of dopants, which are the cations Ca2+, Mg2+, Al3+, Y3+, Sc3+, Al3+, Th4+, Zr4+ and Si4+. The variations are discussed on the basis of the diffusion of cerium vacancies as the rate-limiting step, and on the basis of calculated expressions of the concentrations of oxygen vacancies, electrons and cerium vacancies vs. the oxygen partial pressure and the dopant content. For cations that are smaller than Ce4+, the comparison between the experimental and theoretical rates asserts the validity of the model and allows the prediction of the efficiency of a cation to stabilize the surface area, from its associations with oxygen vacancies and with the electron-bearing species, Ce′Ce.

Characterization of a cerium dioxide powder from its equilibrium with water vapour

Abstract The equilibrium between water and a cerium dioxide powder was studied by means of thermogravimetry between 373 and 573 K. The isotherms can be interpreted by the additive contribution of water adsorbed on two distinct sites which differ in their standard enthalpy of fixation and their relative population. The results agree with the model water is fixed on the ceria surface through direct bonds to cerium atoms and by hydrogen bonding to hydroxyl groups located in the micropores. The latter sites disappear following calcination at high temperature. This is confirmed by transmission electron microscopy photographs and nitrogen adsorption measurements.

Surface area stability of lanthanum-doped ceria

Abstract High surface area ceria was doped with lanthanum ions and calcined at 900 K under various partial pressures of oxygen and water vapour. The kinetic rates of crystallite size increase were observed to vary as P − 1 6 O 2 . By addition of 0.5 and 1% of lanthanum ions, the rates were lowered, and then significantly decreased for 10% (cat.). A model of this process involved association between lanthanum ions and oxygen vacancies in order to explain the better stability of doped ceria.

Thermal stability of (0.15–0.35 wt%) rhodium on low-loaded ceria-supported rhodium catalysts

The thermal stability of rhodium on ceric oxides submitted to high-temperature reduction (773–1173 K) and aging in air at 1173 K was studied by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and benzene hydrogenation. Catalysts were prepared by anionic exchange of rhodium chloro complexes on both high (144 m2 g-1) and low specific surface area ceria (6 m2 g-1). In reducing conditions, both types of catalysts stabilized 5 nm metal rhodium particles. Calcination in air at 1173 K pointed out the interest of rhodium exchanged over a low surface precalcined ceria. In that case, all the metal remained at the surface of CeO2 after calcination whereas 50% of the initial supported metal was buried into sintered ceria for the catalyst prepared from the high-surface CeO2.

Improvement of the thermal stability of ceria support

Abstract The experimental variations of the rate of crystallite growth in ceria at 943 K are reported as a function of the content (≤10 at %) in foreign cations or additives such as Mg 2+ , Al 3+ , Y 3+ , and Si 4+ ions. The process of surface area decrease due to crystallite growth is described by a kinetic model involving oxygen and cerium ions diffusion. From the calculation of the concentration in oxygen vacancies, electrons and cerium vacancies, the diffusion current can be obtained versus the oxygen partial pressure and the foreign cation concentration. The rate obtained from the model is compared to the experimental one.